Abaco (the browser shipped with Plan 9) has support form most of the HTML4 spec, but lacks support for things like CSS and Javascript (see: http://plan9.bell-labs.com/sources/contrib/fgb/abaco.pdf).

It might be feasible to swap out Abaco's HTML parsers and such with a "modern" rendering engine like Gecko or Webkit while retaining its webfs-based approach to navigating the world wide web.